The heart is a remarkable muscle that beats by way of an electrical conduction system. Disease and abnormality in the electrical pathways cause normal sinus rhythm to be disrupted, prompting the heart to beat too slowly, too quickly, or too erratically. This is called a cardiac arrhythmia.
Cardiac Arrhythmia
An arrhythmia occurs when the SA node becomes diseased (sick sinus syndrome), or when the AV node fails to pass an impulse along the heart's electrical pathway, or when there is blockage anywhere in the heart's conduction system. An arrhythmia affects the heart's pumping ability by either speeding up or slowing down the electrical impulses responsible for each heartbeat. If the heart beats too slowly (bradycardia), it will not pump enough blood to meet the body's demands. If the heart beats too quickly (tachycardia), it will not have enough time between contractions to fill completely and will fail to pump enough blood to the body. Sometimes a tachycardia will progress into fibrillation, which is a “quivering” of either the heart's atria or ventricles and an inability to pump at all. Fibrillation, which can also occur spontaneously, causes a serious disruption in the rhythm and function of the heart.
Arrhythmias are further classified according to where they originate: atrial or supraventricular (above the ventricles) arrhythmias and ventricular (in the ventricles) arrhythmias.
Fibrillation
Fibrillation is caused when the heart muscle begins to quiver, or fibrillate, continually and cannot contract normally. When a heart is in a state of fibrillation, there is no synchronization between the atria and the ventricles. This causes the patient to experience a racing sensation—and sometimes discomfort in the chest—and/or to feel light-headed or faint.
Atrial Fibrillation (AF)
Atrial fibrillation (AF or AFib) is a very fast, uncontrolled atrial heart rate caused by rapidly fired signals. During an episode of AF, the atrial heart rate may exceed 350 beats per minute. Not all of these beats reach the ventricles, so the ventricular rate is not this high. However, the ventricular rate is often higher than normal, exceeding 100 beats per minute. Sometimes an impulse will circle the atria, triggering atrial flutter, which is similar to AF. Alone, AF is rarely serious, but if a patient has high blood pressure, valvular disease, or heart muscle damage, AF can increase the risk of stroke or heart failure.
There are several treatments for AF, including medication and electrical cardioversion. Electrical cardioversion converts the heart rate back to normal sinus rhythm through the use of a controlled electrical shock that excites all the heart cells at once, allowing the SA node to resume its role as the heart's natural pacemaker.
Ventricular Fibrillation (VF)
Ventricular fibrillation (VF or VFib) is a chaotic heart rate resulting from multiple areas of the ventricles attempting to control the heart's rhythm. Ventricular fibrillation can occur spontaneously (generally caused by heart disease) or when ventricular tachycardia has persisted too long. When the ventricles fibrillate, they cannot contract normally, hence, they cannot effectively pump blood. The instant VF begins, effective blood pumping stops. VF quickly becomes more erratic, resulting in cardiac arrest or sudden cardiac death. This arrhythmia must be corrected immediately via a shock from an external defibrillator. The defibrillator stops the chaotic electrical activity and restores normal sinus rhythm.
Treatment of Arrhythmias
There are several techniques that physicians use to detect cardiac arrhythmias and, in particular, fibrillations. There are also several treatment options for rhythm management. One such treatment option is the implantation of an implantable cardiac stimulation device (ICSD), such as a pacemaker or implantable cardioverter-defibrillator (ICD).
Implantable Cardiac Therapy Devices (ICTDs)
Pacemakers and ICDs (and other implantable cardiac stimulation device (ICSDs) are common examples of implantable cardiac therapy devices (ICTDs).
Implantable cardiac therapy devices (ICTDs) are implanted within the body of a patient to monitor, regulate, and/or correct heart function. ICTDs include implantable cardiac stimulation devices (e.g., implantable cardiac pacemakers, implantable defibrillators) that apply stimulation therapy to the heart as well as implantable cardiac monitors that monitor heart activity.
ICTDs typically include a control unit positioned within a casing that is implanted into the body and a set of leads that are positioned to impart stimulation and/or monitor heart activity. With improved processor and memory technologies, the control units have become increasingly more sophisticated, allowing them to monitor many types of conditions and apply tailored stimulation therapies in response to those conditions.
ICTDs are typically capable of being programmed remotely by an external programming device, often called a “programmer”. Today, individual ICTDs are equipped with telemetry circuits that communicate with the programmer. One type of programmer utilizes an electromagnetic wand that is placed near the implanted cardiac device to communicate with the implanted device. The wand contains a coil that forms a transformer coupling with the ICTD telemetry circuitry. The wand transmits low frequency signals by varying coil impedance.
Treatments with ICTDs
Depending upon the type and severity of the arrhythmia, physicians may choose from a number of therapies to manage the abnormal rhythm. Those therapies may include the use of an ICTD.
Cardiac Pacemaker
An artificial permanent pacemaker can take the place of a diseased sinus node or it can help the heart beat despite a blockage in the electrical conduction system. One or more leads (a thin, coated wire) are inserted through a vein, and the tip of the lead (called the electrode) is placed in either the atrium or the ventricle, against, or attached to, the heart's lining. The pulse generator, which encases the electronic circuitry of the pacemaker, is attached to the lead and placed just under the skin, usually in the chest.
Implantable Cardioverter Defibrillator
An implantable cardioverter defibrillator (“defibrillator”) is a device much like a pacemaker, with leads and a pulse generator that encloses electronic circuitry. The defibrillator consists of a hybrid (electronic circuitry) and a battery, both of which are enclosed in a sealed titanium capsule. The defibrillator serves as the “brain” of the implanted system, monitoring the heart's activity and generating therapy when appropriate. Defibrillators are high-tech devices that contain sophisticated detection algorithms (to ensure therapy is appropriate and that patients do not receive unnecessary shocks) and tiered therapy options.
Typically, a defibrillator corrects dangerous ventricular arrhythmias which, if left untreated, would result in sudden cardiac death. The device monitors the heart's rhythm and, when it detects a serious fast ventricular arrhythmia, typically administers one of three therapies: pacing (a burst of critically timed low-energy output pulses), cardioversion (low-level shocks delivered at a specific point of the tachyarrhythmia), and/or defibrillation (high-energy shock therapy).
The defibrillator detects fibrillation and delivers electrical therapy-pacing pulses and/or defibrillation therapy. Defibrillator therapy is often prescribed for patients who have generally experienced at least one episode of ventricular tachycardia or ventricular fibrillation, previous cardiac arrest, or drug therapy that was ineffective in controlling the tachyarrhythmia or that caused severe side effects.
Although defibrillators are traditionally and most frequently used to rescue a patient suffering from ventricular fibrillation (VF), some defibrillators are designed to correct atrial fibrillation (AF). AF is typically not fatal; therefore, defibrillation may be delayed. Although an ICTD may rescue AF automatically, most often AF rescue is controlled manual. When a patient recognizes that they suffer from AF, they initiate the AF defibrillation to correct their heart's arrhythmia.
Determination of Defibrillation Shock Energy (DFSE)
After a defibrillator is implanted, it is programmed and tested to ensure the system will effectively correct the patient's arrhythmia. When a defibrillator is implanted into a patient, the medical personal attempt to determine the sufficient shock level (i.e., “sufficient energy level”) to effectively correct the patient's arrhythmia (i.e., rescue the patient). This sufficient shock level is also called the defibrillation shock energy (DFSE).
For each patient in a given situation, their heart has a defibrillation threshold (DFT) characteristic. The DFT is the minimum sufficient shock necessary to successfully rescue the patient.
Hopefully, the therapy provided by the defibrillator at the DFSE level is at or above the DFT of the patient. If so, the therapy will rescue the patient. Therefore, it is desirable for the DFSE to be set above and near the DFT of the patient.
A defibrillator may be programmed with more than one defibrillation shock energy (DFSE) setting. More specifically, there may be one or more ventricular fibrillation DFSE (VF-DFSE) settings and one or more atrial fibrillation DFSE (AF-DFSE) settings. Herein, the general term DFSE, without modification or other context, expressly includes the specific terms AF-DFSE and VF-DFSE.
Similarly, a patient has one or more ventricular fibrillation DFT (VF-DFT) and one or more atrial fibrillation DFT (AF-DFT). Herein, the general term DFT, without modification or other context, expressly includes the specific terms AF-DFT and VF-DFT.
The DFSE may be described in energy level units (e.g., measured in voltage or joules) delivered to the patient. Herein, the DFSE for a specific patient is the set at an energy level that is sufficiently high enough to effectively rescue the patient with a high degree of probability.
For example, in one patient, the VF-DFT may be 5 J (joules) and 200 V (volts), but for another, it may be 20 J or 600 V. A typical range for AF-DFT is about 2–5 J. The DFT range may be large across a patient population. Many variables (including the patient, pharmacological, and lead placement) account for this large range. Consequently, the DFT varies with each person and current conditions.
For example, variables related to a particular fibrillation episode may play a role in the DFT. In one study (Boriani, Biffi, et al., Predictors of Atrial Defibrillation Threshold in Internal Cardioversion, PACE, Vol. 23 (Nov. 2000)), it was suggested that the DFT for a patient suffering from AF was related to many factors associated with the AF episode itself. Two of the primary factors included duration of the AF episode and the rate of the AF.
FIGS. 1A and 1B are charts derived from the above-referenced study. Theses charts illustrate the relationship between these two primary factors on the AF-DFT. FIG. 1A illustrates that the magnitude of the shock necessary to rescue the AF patient increases with the rate of AF cycles of their AF episode. Similarly, FIG. 1B illustrates that the magnitude of the shock necessary to rescue the DFT AF patient increases with the duration of the AF episode (measured in months in this instance).
Optimum DFSE
The optimum DFSE is the smallest amplitude of energy of a defibrillating shock necessary to rescue the patient with a high degree of probability. An improved DFSE is one that approaches the optimum DFSE and may be equivalent to the optimum DFSE. Of course, there may be many of the factors that go into a determination of an improved or optimum level.
Conventional DFSE Determination
Unfortunately, conventional determination of VF-DFSE and AF-DFSE does not attempt to find an improved level. Typically, a defibrillator is programmed to deliver a series of shocks with increasing, but fixed, energy levels. Of course, the defibrillator only delivers as many shocks as is necessary in that series to rescue the patient.
No conventional defibrillator exists that automatically adjusts the DFSE to an improved level based upon historical and present factors. No conventional defibrillator exists that automatically adjusts the AF-DFSE to an improved level based upon historical and present factors (in particular, present fibrillation episode variables). No conventional defibrillator exists that automatically adjusts the VF-DFSE to an improved level based upon historical and present factors.
Accordingly, there is a need for such a defibrillator.